Unfused color toner on transparent sheets has a tendency to scatter projection light rather than transmit it. The scattered light fails to reach the projection screen and the colored toner shows up as a grey image rather than the desired color. The conventional solution is thorough fusing of the color toner which greatly reduces the scattering and improves the projection. However, fusers that are ordinarily designed for use with paper copies do not adequately fuse color toners on transparency stock at the same speed and temperature that they fuse toners on paper.
Off-line lamination of toner images on receiving sheets, both transparent and opaque, has been used to reduce the scattering of images on transparent receiving sheets and protect images on opaque sheets.
United Kingdom Patent Application 2,150,885, published Jul. 10, 1985 suggests laminating toner images on a paper receiving sheet using the fuser of a copier. A transparent film is overlaid the toner image as the receiving sheet is fed into the fuser. The transparent film is a polyester having an acrylic resin which contacts the toner image, the resin has a melting point similar to that of the toner. This laminating process for protecting toner images on paper requires a separate supply mechanism for laminating material and appropriate hardware in the vicinity of the fuser.
Japanese Kokai 88/039439 and 88/039440 suggest putting a laminating sheet in contact with a printed surface of a printing sheet in the supply tray of a copier and feeding it all the way through the copy sheet path of the copier to laminate the two sheets. The two sheets are apparently placed in contact in the supply tray by hand.